1. Field of the Invention
The present invention relates to substrates of information recording disks and apparatus and method for injection molding the same.
2. Description of the Prior Art
Most transparent substrates of information storage disks such as optical disks or the like have been formed by molding material such as PC (polycarbonate) or PMMA (polymethyl methacrylate) through molding with an injection molding apparatus.
FIG. 6 is a schematic sectional view showing a clamped condition of a mold of a conventional apparatus for injection molding information recording disks. The mold of such a conventional apparatus has a fixed mold body 1 and a movable mold body 2 which are arranged as a pair.
The fixed mold body 1 comprises a fixed seat 3, a disk-shaped fixed mirror plate 4 which has a surface for forming one of the surfaces of the substrate to be formed, and a retainer plate 5 for retaining the fixed mirror plate 4 onto the fixed seat 3. A pipe-shaped spool bush 6 is inserted into the fixed mirror plate 4 to extend through the center of the fixed mirror plate 4. A spool 7 ia provided within the spool bush 6 so that when melted molding material is injected from a nozzle 8 of an injection mechanism, the spool 7 directs the melted material into a mold space 9 defined by molding the surfaces of the mold.
On the other hand, the movable mold body 2 is formed of a movable seat 10, a disk-shaped movable mirror plate 11 having a surface for supporting a stamper, and a retaining plate 5 for fixing the movable mirror plate 11 to the movable seat 10. A pipe-shaped inner circumferential clamp 12 and an circumferential outer clamp 13 are provided on the movable mirror plate 11 at the portions thereof corresponding to outer circumference and inner circumference of the stamper. The outer clamp 13 has a molding surface which defines the outer circumference of the substrate to be formed. A cutting mechanism for boring a center hole in the substrate of material which has been cured is provided at the center of the movable mirror plate 11.
The fixed mold body 1 is secured to a certain retaining member (not shown) so that the nozzle 8 of the injection mechanism fits in the spool 7. The movable mold body 2 is connected to a mold clamping means, for example, a hydraulic cylinder mechanism 100 for applying a mold clamping force to the movable mold body 2 to urge the movable mold body against the fixed mold body 1, so that the movable mold body 2 can move in a direction (indicated by arrow S) perpendicular to the opposing molding surfaces of the mold bodies 1 and 2. Further, an injection mechanism (not shown) for injecting melted molding material such as PMMA (polymethyl methacrylate) resin through the nozzle 8 thereof is provided within the mold space defined between the respective opposing molding surfaces of the movable mold body 2 and the fixed mold body 1.
FIG. 7 is a sectional view of the cutting mechanism and its vicinity depicted by a circle A shown in FIG. 6. The stamper 20, which forms part of a molding surface 19 of the movable mirror plate 11, for forming signal recording pits on the substrate to be formed is secured to the movable mirror plate 11 by a force along the outer circumferential portions of the stamper 20 by means of the outer circumferential clamp 13 and, also by a force along the inner circumferential portion of the stamper by the inner circumferential clamp 12, respectively. The inner clamp 12 is supported by a lock bar 21. A groove 22 is provided within the movable mold body 2 so that cooling water passes through. The inner circumferential clamp 12 has an annular protrusion 23 having a generally triangular cross section which protrudes into the mold space 9 and which urges the inner circumference of the stamper 20 against the movable mold body 2 so as to secure the stamper to the latter.
A pipe-shaped punch 24 which is provided at the central portion of the movable mirror surface plate 11 in a manner so that the punch 24 is movable in a direction of an arrow S and is surrounded by the inner clamp 12 which is in the form of a hollow circular cylinder. The tip end of the punch 24 forms part of the aforementioned molding surface, that is, in this case a portion corresponding to a central hole of a substrate which is to be formed by cooling and curing the melted molding material injected therein. The punch 24 is connected to the output shaft of a hydraulic cylinder mechanism (not shown) so that it is driven in the direction of the arrow S. An eject pin 25 is provided within the punch 24 so as to be movable in reciprocating fashion in the direction of the arrow S to thereby release the portion of the molding material corresponding to the central hole that is punched out by the punch. This eject pin 25 is driven by the hydraulic mechanism which is not shown. These elements form the cutting mechanism.
A cylindrical ejector 26 and a cylindrical fixed sleeve 27 are provided between the inner circumferential clamp 12 and the punch 24. The ejector 26 is arranged to be driven by the hydraulic cylinder mechanism so as to be movable in reciprocating fashion in the direction of arrow S to push the molded substrate in order to separate it from the movable mirror plate 11. The fixed sleeve 27 is secured to the movable mold body 2 by means of a bolt or the like and arranged to guide the movement of the ejector 26. An air blow groove 29 is provided on the side surface of the fixed sleeve 27 which faces the inner circumferential clamp 12. The air blow groove 29 communicates with an external gas supply pump 30 through a groove formed in the sleeve 27 through machining in the longitudinal direction of the fixed sleeve 27. When the molded substrate is to be released from the mold, pressurized gas from the gas pump 30 is passed through the air blow groove 29 and is injected through a small gap between the inner circumferential clamp 12 and the fixed sleeve 27 so that the substrate is pushed to separate it from the movable mirror plate 11. These elements form a mold release mechanism.
The fixed mold mirror plate 4 of the fixed mold body 1 has a die portion 40 having an inner circumferential edge which corresponds to the punch 24 of the mold release mechanism forming part of the molding surface. The die portion 40 cooperates with the tip end of the aforementioned punch 24 so as to punch out the central hole of the substrate. The tip end of the punch 24 and the circular spool bush 6 surrounded by the die portion 40 form a runner and a gate. The spool 7 guides the melted molding material injected from the nozzle 8 into the mold space. Additionally, the knockout, that is, the spool bush 6 and the nozzle 8 are retracted prior to protrusion operation of the punch.
Next, the operation of the information recording disk injection molding apparatus will be described hereunder.
First, the fixed mold body 1 is firmly urged against the movable mold body 2 so as to perform mold clamping with the mold space 9 formed in the mold. Subsequently, the melted molding material is injected and guided into the mold space 9 by the spool 7. Then, the melted molding material is cooled and cured to thereby form a molded substrate.
Thereafter, the punch 24 is protruded by a predetermined distance. By operating the punch 24 in this manner, the spool bush 6 and the nozzle 8 are retracted together with the center hole portion which is punched out, whereby the central hole is formed in the substrate.
As depicted by an arrow in FIG. 8, the movable mold body 2 is then moved sufficiently away from the fixed mold body 1 sufficient and the eject pin 25 protrudes to push out the spool-forming portion 50 from the punch. FIG. 8 is an enlarged sectional view of the mold release mechanism and the cutting mechanism in a portion indicated by a circle B in FIG. 7, in the mold release step.
Next, in the mold release step, a substrate 64 is released from the movable mold body 2 by means of the pressurized gas supplied through the air blow groove 29. The substrate 64 is then separated from the movable mold body 2 by means of the ejector 26 which protrudes thereto, so as to be collected.
Thus, such a substrate 64 as shown in FIG. 9 is obtained. In the main surface of the substrate 64, an annular groove 64c corresponding to the annular protrusion 23 is formed between a central hole portion 64a and an information recording region 64b.
Many of the substrates manufactured in this manner have been defective because the linear patterns as designated by a reference numeral 64d, called a molding stain which has a different color than the remainder of the substrate portion, may occur in the main surfaces of the substrate 64 obtained in this fashion. Further, the flow, orientation, and pressure distribution of the molding material are not fixed, and therefore it is difficult to make the birefringence of the substrate uniform.
Further, with the conventional apparatus for injection molding a substrate of an information recording disk, the pressurized gas for separating the molded substrate 64 from the stamper 20 flows to outside, as shown by an arrow in FIG. 8, through the central hole portion which has been punched out. Thus the pressurized gas is not distributed uniformly over the enter surface of the substrate 64, causing the substrate 64 and the mold to separate non-uniformly. Further, there is a shortcoming that if the inner and outer circumferential portions are separated from the mold at different times, the pits formed on the disk may be damaged.